This invention relates to a method of manufacturing continuously chirped, gain flattening filters in optical fiber and waveguide passive components for applications such as telecommunications and sensors.
In Optical Communications, passive components are needed to format the transmitted light signals. An opto-electronic component, so as an optical filter, must achieve a very specific filter shape. The filter shape is defined as the relationship between the reflectivity and the wavelength within the filtered band. Each filter shape may be designed or fabricated for a particular application.
The Erbium Doped Fiber Amplifier (EDFA) was introduced to provide optical amplification for many separate light signals. The EDFA transmits and periodically regenerates optical light signals. The singular use of the EDFA in an optical network poses a problem since the amplifier gain of an EDFA, is not constant across its useful amplification spectrum. Gain Flattening Filters (GFF) were developed to counter balance the transmission characteristics of the EDFA. The GFF coupled to the EDFA produces a flattened gain across the amplifier band.
In the past, the GFF was fabricated using a myriad of techniques. The most common techniques being the use of thin film coating, planar waveguides, or the concatenation of many Fiber Bragg Gratings (FBG). The above-mentioned techniques have proved to be insufficient as performance characteristics, such as insertion loss, filter shape accuracy, and temperature stability, were inadequate. A GFF with 5 to 6 dB peak attenuation produced a filter shape with an accuracy no lower than 0.5 dB.
U.S. Pat. No. 5,367,588, granted to Hill et al., discloses a method of fabricating Bragg Gratings using a silica glass phase grating mask The silica glass phase grating mask is positioned between a laser producing a UV light beam and in close proximity to the optical waveguide. Laser irradiation through the phase grating mask results in an interference pattern which is imprinted (photo-induced) into the core of the optical waveguide. The photo-induced refractive index modulation forms a fiber Bragg grating which selectively reflects light of specific wavelengths, depending on the modulation periodicity.
The phase mask is described as a one-dimensional surface relief structure fabricated in a flat substrate of high quality fused silica transparent to the laser irradiation. The laser irradiation through the phase mask may produce periodic and aperiodic variation in the index of refraction in the optical waveguide depending on the periodicity of the mask. A chirped FBG refers to a variable pitch in a particular grating. Chirping causes wavelengths within an optical fiber to be reflected according to the varying pitch of the grating.
The present invention seeks to provide a GFF with a technique amenable to manufacturing in large volume and having a filter with a more precise control of the spectral response.
The present invention involves the use of a single Bragg Grating to cover the entire wavelength band to be filtered. A Bragg Grating may be described as a periodic or aperiodic perturbation of the effective refractive index of an optical waveguide. Essentially, a Bragg Grating may reflect a predetermined band of wavelengths of light incident on the grating, while passing all other wavelengths of light.
Bragg Gratings are photo-imprinted into a photosensitive optical waveguide material. Photo-imprinting the optical waveguide material involves the irradiation of an optical waveguide material in order to change the refractive index of the core of the optical waveguide. Under ideal conditions, irradiation of the optical waveguide with an interference pattern of light causes a permanent index modulation induced in the core of the optical waveguide. The end result is a grating that is photo-imprinted in the core of the optical waveguide. A Fiber Bragg Grating (FBG) is a narrowband filter permanently imprinted into the core of the optical fiber.
The present invention provides a method for producing continuously chirped Gain Flattening Filters (GFF) in a single Bragg Grating. The method utilizes a phase mask which is chirped continuously along its length to cover the entire GFF band and an amplitude mask. In a first step, an approximate GFF profile is formed by irradiating through both a continuously chirped phase mask and an amplitude mask into the core of the optical waveguide. The amplitude mask is used to control the amount of light reaching each section of the phase mask and thus to control the attenuation of the various wavelengths within the band. In a subsequent step, each particular wavelength region within the filter is locally irradiated to achieve a more precise spectral response from the GFF. The UV light beam may be adjusted and positioned to focus on a particular wavelength region of the filtered band. Through the strong chirp of the phase mask, there is a direct relationship betwen wavelength and position along the grating length.
In a first aspect, the invention provides a method of making a continuously chirped gain flattening filter in a single Bragg grating in an optical waveguide material , including the steps of:
disposing a strongly chirped phase mask placed between a light beam and the optical waveguide material, the light beam being capable of changing the effective index of refraction of the optical waveguide material, and
irradiating said optical waveguide material with said light beam non-uniformly through the phase mask, said irradiation producing a suitable filter response and required attenuation over the filter band.
The non-uniform irradiation may be provided by an amplitude mask in the light beam, a narrow slit positioned along different parts of the light beam for different amounts of time, or a combination of both.